How Are Building Occupant Loads Determined? | ARTiculations


Have you ever seen one of those signs indicating
the occupant capacity of a room and thought, how exactly do they get that number? If you thought it was how many people could
be physically packed into the room shoulder to shoulder then, no that’s most likely
not it. Occupant loads are, for the most part, determined
by: means of egress, which basically means the design of exiting components throughout
the space, as well as the number of available sanitary fixtures such as toilets and lavatories. So how are these components designed to meet
the intended occupant load? Egress is arguably one of the most important
aspects of architectural and interior design. The exiting requirements for proper and safe
evacuation depends largely on what the space is being used for. For example a movie theatre will usually hold
more people compared to a typical office of the same size, and a storage room carrying
flammable liquids is more dangerous than a typical residential dwelling. Building codes also differ depending on where
you are. Here in North America, most states, provinces
and municipalities will adopt “model building codes,” such as the International Building
Code, or the National Building Code of Canada. For this video I will refer mostly to the
IBC. In order to determine egress provisions. We need to figure out how many people are
expected to use a space. In a space that only contains fixed seats,
like a movie theatre, it’s pretty straight forward to figure that out. It can be done by counting the number of seats. But what about spaces without fixed seating? Well, another method is to use the “occupant
load factor” table provided in the building code. For example, the load factor of an exhibit
gallery or museum is 30 sqft/occupant. So in a 3000 sqft room the occupant load for
an art gallery would be 100. In Business Areas, such an office, the load
factor is 150 sqft/occupant. So in the same space the occupant load for
an office would be 20. The load factor table, however, does not restrict
the space from having more occupants. The 3,000 sqft gallery can be designed for
more than 100 people, as long as the requirements for egress, life safety, sanitary fixtures
and other code mandates are satisfied. The more difficult thing to do is designing
the space for less occupants than what’s specified by the load table. So if you’re trying to argue what this 3000
sqft gallery will only be used by a maximum of 25 people at any given time, then you need
to make a good case to the local building official and they would have to approve it. So let’s talk exits. Or more accurately – means of egress. What exactly is a means of egress? Well the IBC defines it as “a continuous
and unobstructed path of vertical and horizontal egress travel from any occupied portion of
a building or structure to a public way.” The reason we talk about it in this way is
because exiting a building is not a one-step event, it’s a process that involves multiple
exiting components. So, pretty obvious – every space needs at
least one means of egress. But two or more means of egress are required
if the space exceeds a certain occupant load under certain occupancy classifications. Occupancy classifications are formal designations
for the primary purpose of the building or portion of building. Since Art Galleries normally fall under the
Assembly occupancy, we would look under the requirements for Occupancy A, which indicates
that if the space only has one means of egress, the occupant load cannot exceed 49. In our case of 100 occupants, we would have
to provide two means of egress. Exits, or access to exits, also have to be
positioned and spaced accordingly. Because, say there is a fire blocking the
path to one doorway, there needs to be an alternative way to get out. You can’t just have two doors next to each
other and call that two exits. They have to be certain distances apart. And the distances are based on ratios and
formulas outlined by the code which may vary depending on if the building is sprinklered
or not. The means of egress also have to be large
enough to handle the occupant load. For example, doorways and corridors along
a means of egress must have a clear headroom of 80” high, have a minimum clear width
of 32” for doors, 36” for corridors 44″ for corridors serving an occupant load of
50 or more, or 0.2” multiplied by the occupant load, whichever is more. These width capacities may be reduced in certain
occupancies if the space is equipped with an automatic sprinkler system. Going back to our art gallery that require
two means of egress. Let’s say they are exit access doorways. Since 0.2” x 100 is only 20”. In this case, both doors have to be at least
32” wide each. This is not necessarily the size of the door
though, it’s actually the clear opening between any protruding hardware when the door
is opened at 90 degrees. So often a 36” door needs to be used to
achieve a 32” opening. Let’s take a look at an example with a higher
occupant load. For instance a community meeting room with
loose seating. This usage, as per the load factor table is
listed as 7 sqft/occupant. So in our 3000 sqft room we could expect 429
people. Which when multiplied by 0.2” would equal
85.8”. Since we definitely need at least two exits
or exit access doorways, one way to achieve this is by having two doors, each having a
clear width of a least 42.9”. In High Hazard occupancies, and in all other
occupancies where the occupant load of the space is 50 or more, the door also must swing
in the direction of exit travel. Occupant load above certain capacities may
also trigger life safety and fire protection requirements, such as requirements for automatic
sprinklers and fire alarm systems. But while life safety and means of egress
are very important building components, so are personal hygiene and access to toilets. Thus building codes mandate a minimum number
of toilets, lavatories, drinking fountains, service sinks, and sometimes also bathtubs
and showers. As per IBC, In the office area, we would need
to provide 1 water closet for every 25 persons of each sex for the first 50 people. Since we only have 20 people, that’s 10
males and 10 females, we could provide one restroom per sex. However, there is an exception in the code
where you do not need to provide a separate facility for each sex in business occupancies
of 25 or less. So in this case, providing one restroom total
is acceptable. In addition, based on our occupant load, a
minimum of one lavatory, one drinking fountain and one service sink are also required. In the community meeting room of 429 people,
the requirement is one water closet for every 65 females and 1 for every 125 males. Since we have about 215 of each sex, this
means we need four water closets for women and two for men. In the female restroom this would be four
toilets. In the male restroom, up to half of toilets
can be urinals. In case you’re wondering why women get more
toilets than men, this is due to the fact that biologically it takes longer for women
to use the restroom. Additionally one lavatory is required for
every 200 people of each sex, this means we need two lavatories for both restrooms. And, one drinking fountain and one service
sink are also required. Essentially, every built environment needs
to be designed to allow its occupants to exit safely in a timely manner in the event of
an emergency, and provide adequate toilet and personal hygiene facilities for its occupants. While the examples I’ve given in this video
are relatively simple, there are many more complex requirements. Plus, real world environments are generally
not as straight forward, and often deal with existing construction, mixed occupancies and
municipal regulations. Thus, most of the time, load calculations
and code reviews must be carried out by a licensed interior designer or architect. I find that a lot of people perceive creative
design to be dominated by form and aesthetics. When in reality, form is often secondary to
function, and design can be quite a constrained exercise with a lot of limitations. However, personally I think working with constrained
parameters is even more interesting and, as a matter of fact, requires even more creativity. Thanks for watching. This episode is a part of a series where I
explore various aspects of the interior design profession. If you liked it, then you may enjoy some of
these as well. And please subscribe to keep watching. Until next time! *snap!*

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